Craig Shallahamer's Blog
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The text below is an except from the book,
Oracle Performance Firefighting, written by
Craig Shallahamer of
OraPub, Inc.
Figures and tables are not included on this page, only their reference.
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©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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<p>The second section is the response time summary. This provides a high-level view of the response time components. In Figure 2-15, the total service time is 10 seconds of CPU, and the total wait time is 621 seconds. Written another way, during the 120-second interval, Oracle processes consumed only 10 seconds of CPU time and waited 621 seconds. The wait time is classified into IO time and non-IO time. In Figure 2-15, you can see the IO wait time is 615 seconds and the non-IO wait time is only 6 seconds. Clearly, Oracle processes are spending a significant portion of their time waiting for blocks outside the Oracle buffer cache!
p><p>A key statistic shown in the rtsysx.sql report is the Oracle CPU utilization consumption. During the 120-second interval, Oracle processes consumed only 8% of the total CPU available. This statistic is simply the Oracle CPU consumption divided by the host CPU time capacity. Figure 2-15 shows that Oracle processes consumed only 10 seconds of CPU. The total CPU capacity the host can provide is the number of CPU cores multiplied by the report interval. In this case, there is a single CPU core and the interval is 120 seconds, resulting in a total CPU capacity available of only 120 seconds. Therefore, the percentage of Oracle CPU consumed is 8%, or 10/(1*120), which is 8.33%. This simple statistic gives us a surprisingly useful understanding of this instance's host CPU impact. If this is the only instance running on the host, it also provides an operating system CPU utilization picture, without executing an operating system command. In this situation, this is the only instance; therefore, we would expect the operating system CPU utilization to be a few percentage points higher than 8%.
p><p>It may seem odd that a 120-second interval can show 621 seconds of wait time. This is very common in Oracle systems. The 621 seconds of wait time are the total wait time for all sessions during the 120-second interval. For example, if there were 1,000 sessions, and each session waited 1 second during the 120-second interval, then the total wait time would be 1,000 seconds. If 2,000 sessions each waited 2 seconds, then the total wait time would be 4,000 seconds. The more time processes wait, the more wait time. So one way to increase the wait time is to keep increasing the workload, which can be represented by the number of sessions or the work the sessions are doing.
p>
©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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